The use of conversion layers for increasing the protective action of cathodic corrosion protection systems and as primer for varnishes and paints has been known for a long time. In addition to phosphating methods, the method of chromating the surfaces has become established, especially for zinc-, aluminium- and magnesium-containing substrates.
Here, the surface to be treated is exposed to a treatment solution whose essential constituents are chromium(VI) compounds. The conversion layer produced therefore also contains chromium(VI) ions. Chromating layers generally display good corrosion protection and good decorative properties. A disadvantage of the use of chromium(VI)-containing solutions or chromium(VI)-containing coatings is the toxicological properties of chromium(VI) compounds. The use of chromium(VI)-containing conversion layers is therefore greatly restricted by, for example, the EC Directive 2000/53/EC (EC old vehicle directive).
As an alternative to chromating solutions, chromium(III)-containing, acid treatment solutions, which in contrast to chromatings are generally referred to as “passivations” or “passivation solutions”, have been proposed. These treatment solutions consist, as proposed, for example, in DE 196 15 664 A1, essentially of a chromium(III) salt in mineral acid solution, a dicarboxylic acid or hydroxycarboxylic acid and a cobalt salt. Such processes known as “thick film passivations” are employed at elevated temperature, about 40-60° C., to achieve a passivation layer thickness sufficient for corrosion protection on zinc surfaces. The necessity of carrying out the process at a temperature above room temperature results from the great lack of reactivity characteristic of chromium(III) ions in contrast to chromium(VI) ions. A significant increase in the reaction times as an alternative to an increase in the temperature is generally not feasible for economic reasons.
As an alternative, Cr(VI)-free black conversion layers can be produced on zinc-nickel alloy layers by treatment with acidic, chromium(III)-containing solutions which additionally contain oxo acids of phosphorus, as described in U.S. Pat. No. 5,415,702. In this process, homogeneous black conversion layers having good decorative properties are formed.
WO 03/05429 describes a similar conversion layer which is likewise produced using a chromium(III)-containing, acidic treatment solution which additionally contains phosphate ions. This surface, too, has good decorative properties but without further after-treatment steps such as sealing does not achieve satisfactory corrosion protection properties.
EP 1 484 432 A1 describes chromium(III)-containing black passivation solutions containing chromium(III) ions and nitrate and also carboxylic acids such as tartaric acid, maleic acid, oxalic acid, succinic acid, citric acid, malonic acid or adipic acid for zinc alloy surfaces. To improve the corrosion protection, the surfaces treated with the solutions have to be subjected to subsequent sealing. The treatment solutions are employed at temperatures above normal room temperature.
US 2004/0156999 likewise describes a process for the black passivation of zinc alloy surfaces. The treatment solutions contain chromium(III) ions and phosphorus-containing anions and also nitrate and an organic carboxylic acid. Examples given for the organic carboxylic acids are citric acid, tartaric acid, maleic acid, glyceric acid, lactic acid, glycolic acid, malonic acid, succinic acid, oxalic acid and glutaric acid.
Essentially, the following process steps are carried out in the production of the protective layers: activation of the metal surface, production of the conversion layer in an acidic passivation bath, removal of the aggressive passivation bath liquid from the surface by means of one or more rinsing operations, drying and application of a further sealer or after-dipping in an after-dipping solution and drying.
This procedure has the disadvantage that rinsing wastewater contaminated with considerable quantities of heavy metals and possibly complexing agents are formed and have to be disposed of or worked up in a complicated and costly manner.
Owing to these disadvantages, efforts have recently been made to develop “no rinse” processes. Here, conversion layers which contain a metal component which increases the passivation, e.g. chromium, titanium or zirconium, and also a film-forming component composed of an organic polymer are essentially applied. These processes are used predominantly for the treatment of aluminium surfaces. “No rinse” phosphating processes in combination with film-forming polymers are also frequently described.
Thus, for example, the document U.S. Pat. No. 6,117,251 describes an aqueous solution containing zinc oxide, phosphoric acid, polyvinyl alcohol and a further metal salt for zinc-phosphating of metal surfaces.
These processes have only very limited use for surfaces of zinc, aluminium and magnesium since although they form films embedded in an organic matrix, no closed chromium oxide layer or surface metal mixed oxide layer, as is necessary for satisfactory corrosion protection, is formed.